igig] VESTAL— PIIYTOGEOGRAPIIY OF COLORADO 191 



situations such as draws and seepage areas of slopes. It in turn is 

 frequently displaced by taller woody vegetation. 



Rock- pine association. — Pinus scopidorum is the important tree 

 of the foothills. It ranges into very variable habitats, and is 

 structurally variable in accordance. It forms infrequent close 

 stands, but in most places is scattered, the ground between the 

 trees being occupied by foothills mixed grassland, Ceanothus, 

 ArctostapJiylos, etc. It is frequent in rocky crests, etc., in the 

 mountain-front, except in the south, where it is commonly replaced 

 by piny on. It extends very locally into the plains in broken 

 country, on butte-crests, etc., and on the elevated Platte-Arkansas 

 divide. 



Pinyon-cedar association. — Pinus edulis and Juniperus mono- 

 sperma are important xerophytic conifers of the southern mountain- 

 front and lower foothills north to the Garden of the Gods, and 

 extending into the southern plains on mesa-crests, canyon-walls, 

 and bluffs of broad valleys. The soil is usually rocky or gravelly. 

 The trees are low and rounded, and do not form a closed assemblage. 



Pseudotsuga association. — Pseudotsuga mucronata forms the 

 mesophytic or relatively mesophytic coniferous forest of the region, 

 and is confined to sheltered ravines and steep north slopes in the 

 foothills. It is infrequent at the mountain-front. 



Oak association. — Small trees of the white-oak group, of uncer- 

 tain taxonomic affinity, form dense copses or open woods in the 

 lower foothills and in the mountain-front about as far north as 

 Platte Canyon. In places grazing destroys the oak slowly and 

 allows increase of grassland. The undergrowth of mesophytic 

 oak areas is much like that of the canyon forest. 



Populus-Salix association. — In stream-side areas of the foothills 

 Populus angustifolia and 4 or 5 common willow species are frequent. 

 Outside the mountains Populus Sargentii, and in the south 

 Populus Wislizeni, replace the narrow-leaf cottonwood. Cotton- 

 woods extend eastward into the plains for many miles along 

 watercourses. 



Canyon forest. — The deciduous trees of the foothill canyons and 

 of ravines, etc., in the mountain-front, include Alnus tenuifolia, 

 Betula fontinalis (these two common along mountain streams), 



